Today a significant percentage of the production equipment is not installed on the surface of the sea, but on the sea bottom. As with all equipment, the equipment on the sea bottom needs regular maintenance. Specifically, during the lifetime of an oilfield, the bore holes and the oilfield itself need maintenance to keep the production as high as possible.
Maintenance of the oil field and the production equipment on the sea bottom is a difficult task that is both time intensive and very expensive.
To perform this maintenance, special vessels are typically needed. Some of the special vessels are known as semi-submersibles and drill ships. These ships have a number of disadvantages. The main disadvantages are their low transit speeds and high daily running cost.
New builds or converted non-dedicated ships, so-called “well intervention vessels”, are increasingly being used to install equipment on the sea bottom and to perform maintenance. The main advantages of small ships are low running cost and acceptable transit speeds. The disadvantage is that these small ships tend to have bad motion characteristics. The small ships move a lot more compared to the bigger units thereby limiting their use to only “good weather windows”.
Well intervention involves everything from lowering a ROV to do a visual check to lowering entire production or maintenance units to the sea bottom and retrieving the units. During the intervention operation, units have to be moved over the deck of the vessel from and to storage areas, the moonpool, and maintenance areas. Often these units are big and heavy and handling them are difficult and dangerous tasks. Sometimes these modules are required to be stacked on top of each other prior to lowering them to the seabed. Often crewmembers have to work on elevated levels to be able to reach all parts of the units. Current practice is the use of man-riding winches. Again, this is both dangerous and time consuming. Many accidents have occurred with the use of man riding winches.
Moving heavy objects also requires the use of cranes. Moving and lifting modules on a moving deck can be quite dangerous and numerous accidents have occurred during this kind of activities.
Apart from moving objects on the decks, lowering and lifting of the units through moonpools located in the vessel creates some specific problems. When lowering units through the moonpool, the objects tend to swing form side to side. Considerable risk of damage to the unit or the vessel arises when the modules are not constrained in some way.
Retrieving objects through the moonpool is equally dangerous. The relative motion of the vessel and the modules can be such that there is also the danger of the module hitting the vessel and thereby endangering the vessel and the lives for the crew.
According to prior art, standard drilling derricks are used in well intervention. The standard drilling derricks have an inverted U shape to lower to and lift objects of the seabed. This shape severely limits the size of the modules that can be handled since every module has to pass through the V-door of the drilling derrick. The two vertical support structures on most standard vessels severely limit the area that can be reached by other cranes and equipment of the vessel.
Due to the construction of the drilling derricks, the drilling derricks must be placed at specific locations in order not to hinder other equipment. This restriction limits the freedom in the design of the vessel considerably. Also, removing the drilling derrick from the vessel when it is not used is a difficult task due to the size and the weight of the drilling derrick.
A need, therefore exists for a module handling system for a well intervention vessel that can be removable mounted on a vessel; has a large freedom of placement on the vessel; does not claim a large working space; can safely move heavy and large objects around the deck; can lower and retrieve modules from the seabed through the moonpool; allows work on the modules on elevated levels safely; and allows modules to be placed on the seabed accurately.
The object of the current invention is to address the problems in the prior art and provide a tower for a monohull with a substantially hollow mast and at least one hoisting device.